(1) Field of the Invention
The present invention generally relates to an evaporated fuel purge control apparatus, and more particularly to an apparatus for controlling a flow of evaporated fuel being fed from a canister into an intake passage of an internal combustion engine through a purge control valve arranged in a purge passage between the canister and the intake passage.
(2) Description of the Related Art
In an internal combustion engine, an evaporated fuel purge control apparatus is provided. In this evaporated fuel purge control apparatus, evaporated fuel from a fuel tank is absorbed by an absorbent in a canister, and the fuel vapor is fed from the canister into an intake passage of the engine through a purge control valve arranged in a purge passage between the canister and the intake passage. The flow of evaporated fuel from the canister into the intake passage is controlled by the purge control valve in accordance with an operating condition of the engine.
Japanese Laid-Open Patent Publication No.61-19962 discloses an evaporated fuel purge control device in which a purge control valve is arranged in a passage between a canister and an intake passage of an engine. The purge control valve is switched on and off in accordance with a duty ratio of an on-time of the valve within a duty cycle to a total duty-cycle time. In the control device disclosed in the above mentioned publication, an intake air amount when the engine is operating is detected, and the duty ratio of the purge control valve is controlled so as to be proportional to the detected intake air amount, in order to prevent the air-fuel ratio from significantly deviating from a desired air-fuel ratio due to the purge fuel flow.
However, the evaporated fuel purge control apparatus has a flow resistance within the system, and there is a time difference between when the internal pressure of the canister changes in response to a purge flow rate change and when the internal pressure of the fuel tank changes in response to the purge flow rate change. If, for example, the purge flow rate changes to a greater value when almost no fuel vapor is stored in the canister, the internal pressure of the canister is lowered. A large amount of fuel vapor is then supplied from the fuel tank to the canister and such fuel vapor is absorbed in the canister. The fuel vapor stored in the canister is subsequently fed into the intake passage, so that the fuel mixture supplied to the engine will be rich.
Conversely, if the purge flow rate changes to a smaller value, a certain mount of fuel vapor returns from the canister back to the fuel tank, and the remaining fuel vapor in the canister is fed into the intake passage so that the fuel mixture supplied to the engine will be lean.
Therefore, the conventional device mentioned above has a problem in that the fuel vapor concentration becomes unstable due to the purge flow rate change, and it is likely that a turbulence of the air-fuel ratio will occur when the purge flow rate changes.
Japanese Laid-Open Patent Publication No.4-72453 discloses an evaporated fuel purge control device in which a solenoid valve is arranged in a purge passage between a canister and an intake passage of an engine. The solenoid valve is switched on and off by a control part in accordance with a duty ratio of an on-time of the valve within a duty cycle to a total duty-cycle time so as to obtain an appropriate level of the purge flow rate in accordance with an operating condition of the engine.
In the control device disclosed in the above mentioned publication, a purge control process to determine the duty ratio for controlling the switching operation of the solenoid valve is carried out by maintaining a purge ratio (=(purge flow rate)/(intake air amount)) at a constant level. This purge control process is called a purge ratio control process. The ratio of a purge fuel amount (supplied from the canister to the engine) to a total fuel mixture amount (supplied to the engine) is constant and thus calculable, and it is easy to determine the purge fuel amount from this ratio. An air-fuel ratio feedback control process is performed to control the air-fuel ratio by changing the purge fuel amount in accordance with the total fuel mixture amount, which prevents the air-fuel ratio from significantly deviating from a desired value.
However, when the canister containing an absorbent for absorbing fuel vapor supplied from a fuel tank is saturated with fuel vapor, and when the engine is operating under an idling condition, it is difficult for the above mentioned conventional device to obtain an adequate level of the purge flow rate. Also, when the engine is operating under the idling condition, the intake air amount is small. In addition, the on-time of the solenoid valve within a duty cycle is small at such a time, and the actual purge flow rate becomes unstable and deviates from the desired purge flow rate due to the flow resistance of the canister. And, it is likely that the air-fuel ratio will become turbulent when the canister is saturated with fuel vapor and the engine is operating under the idling condition.